Niemcewicz was upset when Kościuszko decamped for Europe without giving him any notice.[3]

After Napoleon's 1807 invasion of Poland, Niemcewicz returned to Warsaw and was made secretary of the senate.[2] After the Congress of Vienna, he was secretary of state and president of the constitutional committee in Poland.

As a writer, Niemcewicz tried many styles of composition. His political comedy, The Return of the Deputy (1790), enjoyed great acclaim. His novel, John of Tenczyn (1825), written in the style of Sir Walter Scott, gives a vigorous picture of old Poland.

He also wrote a History of the Reign of Sigismund III (3 volumes, 1819) and a collection of memoirs for ancient Polish history (6 volumes, 1822–23).

Niemcewicz's 1817 pamphlet Rok 3333 czyli sen niesłychany (The Year 3333, or an Incredible Dream), first published posthumously in 1858, describes a Poland transformed into a sinister Judeo-Polonia. The pamphlet has been described as "the first Polish work to develop on a large scale the concept of an organized Jewish conspiracy directly threatening the existing social structure."[4]

His collected works were published in 47 volumes at Leipzig in 1838-40.